Currently, the world is facing a trend that crude oils turn to heavy and poor in quality increasingly, and requirement for heavy fuel oils is decreased down gradually, while requirement for light oils is increasing greatly. Therefore, refinery enterprises are pursuing the maximization of converting residue.
In various processes of lightening residue, it is a good process that residue is hydrotreated in advance, and then the hydrogenated tail oil obtained is further processed by catalytic cracking. After being hydrotreated to remove impurities such as metal, sulfur, nitrogen and the like, the residue has an increased hydrogen content and can be used as a superior-quality feedstock for heavy oil catalytic cracking, making the residue converted completely. Therefore, the process using the hydrogenated tail oil of residue directly as a feedstock for catalytic cracking of heavy oil is now in widespread application increasingly. However, in the combined process, part or all of the catalytic cracking heavy oils remained after separating out catalytic cracking diesel oil, such as, heavy cycle oil, decanted oil and the like, is usually circulated to the catalytic cracking unit to be treated further. However, as heavy cycle oils, decanted oils and the like contain polycyclic aromatics, the yield of light oil is lower, the coking quantity is higher, so that the load of the regenerator is increased, and the treatment capacity and economic benefit of the catalytic cracking unit of heavy oil are decreased. In addition, the heavy cycle oil has a sulfur content about one more time higher than the hydrogenated tail oil, the circulation of the heavy cycle oil also results in increasing sulfur content of products and sulfur emissions.
U.S. Pat. No. 4,713,221 discloses that, on the basis of the combination of conventional residue hydrogenating and catalytic cracking, catalytically cracked (including gas oil catalytic cracking and residue catalytic cracking) heavy cycle oil is circulated to residue hydrogenation unit, and mixed with a topped crude oil to carry out hydrogenation reaction; the hydrogenated residue obtained is introduced into catalytic cracking unit. This small variation may increase the refinery net benefit by 0.29 dollars per barrel of crude oil processed.
CN 1119397C discloses a combined process for hydrotreating-catalytic cracking of residue, comprising: introducing residue and decanted oil together into residue hydrotreating unit to carry out hydrogenation reaction in the presence of hydrogen gas and hydrogenation catalysts; introducing the hydrogenated residue obtained into catalytic cracking unit to carry out cracking reaction in the presence of cracking catalyst, and circulating the heavy cycle oil in the catalytic cracking unit; and separating the slurry oil obtained in a separator to obtain the decanted oil that is returned to the hydrogenation unit.
CN 1165601C discloses a process for combing residue hydrotreating and heavy oil catalytic cracking, comprising: introducing residue and distillate of slurry oil, catalytic cracking heavy cycle oil and optional distillate oil into hydrotreating unit to carry out hydrogenation reaction in the presence of hydrogen gas and hydrogenation catalysts; introducing the hydrogenated residue after separating gasoline and diesel oil, and optional vacuum gas oil into catalytic cracking unit to carry out cracking reaction in the presence of cracking catalyst; introducing the heavy cycle oil obtained into the residue hydrogenation unit, and distilling the slurry oil to obtain a distillate that is returned to the hydrogenation unit.
The aforementioned processes may overcome in a certain extent deficiency existing when the partial or complete catalytic cracking heavy oils, such as heavy cycle oil and decanted oil and the like, that are remained after separating out catalytic cracking diesel oil, are directly circulated to the catalytic cracking unit to be treated further. There are still problems for the hydrogenation unit, such as, poor operational stability and like.